1. Field of the Invention
The present invention relates to a field of radio communications. More particularly, the present invention relates to a signal detection apparatus used in a radio receiver, and an apparatus and a method for estimating a noise power used for signal detection.
2. Description of the Related Art
In the field of the radio communications, research and development are being conducted for realizing large-capacity high-speed information communications of current and the next generation or later. Especially, the Multi Input Multi Output (MIMO) scheme for increasing the communication capacity is receiving attention.
FIG. 1 is a schematic diagram of a communication system of the MIMO scheme including a transmitter 102 and a receiver 104. In the MIMO scheme, different signals are transmitted from plural transmission antennas 106-1˜N at the same time with the same frequency. These transmission signals are received by plural receiving antennas 108-1˜N. For the sake of simplicity, each number of the transmission antennas and the receiving antennas is N, but the numbers may be different between the transmitter and the receiver.
FIG. 2 shows a part relating to signal separation in the receiver 104. Roughly speaking, the receiver receives signals transmitted from the plural transmission antennas with the plural receiving antennas, a signal detection part detects the transmission signals, and the transmission signals are separated to signals for each transmission antenna. The signal separation is performed by signal processing in the two-dimensional frequency domain by using the Minimum Mean Square Error (MMSE) method. Received signals (r) received by each receiving antenna are supplied to a channel estimation part 202. The channel estimation part 202 obtains channel impulse responses or channel estimation values between the transmission antennas and the receiving antennas. The channel estimation result is supplied to a fast Fourier transform part (FFT) 204 to be converted to information in the frequency domain and supplied to a weight generation part 206. A weight W generated in the weight generation part 206 is represented by a following equation, for example:W=(HHH+σ2I)−1H   (1)wherein, “H” indicates a channel matrix having channel impulse responses as its matrix elements, “I” indicates a unit matrix, and a σ2 indicates a noise power arising in the receiver. The superscript “H” indicates transposed conjugate.
The received signals (r) are also supplied to a fast Fourier transform part 210, and are converted to signals in the frequency domain so that the signals are supplied to a MMSE equalizing part 208. The MMSE equalizing part 208 substantially performs signal separation by multiplying the received signals in the frequency domain by a weight WH. The separated signals are supplied to an inverse fast Fourier transform part 212 so that the signals are converted to signals in the time domain, and the signals are output as estimated signals t that are separated for each transmission antenna.
Japanese Laid-Open Patent Application No. 2003-124907 discloses using a signal-to-noise ratio in the MIMO scheme.
For correctly estimating the transmission signals, it is necessary to perform signal separation with very high precision in the signal detection part. For this purpose, it is necessary to correctly obtain the weight W. As shown in the equation (1), since the weight W is largely affected by the channel matrix, the channel estimation needs to be performed correctly in the channel estimation part 202. In addition, according to the equation (1), the weight W is affected by the noise power σ, the noise power needs to be obtained correctly. However, according to the conventional technology of this field, little attempt had been made to obtain the noise power correctly. However, in future products for high-capacity and high-speed information transmission, there is a risk in that signal separation is not properly performed due to lack of estimation accuracy of the noise power.